rt2x00: rt2800: update initial SIFS values

Currently the CCK and OFDM SIFS value is set to 32us. This value is neither
used by the Ralink driver nor specified in 802.11.

Instead of using 10us for CCK SIFS (as defined in 802.11) use 16us like in the
Ralink drivers. And indeed using a SIFS value of 10us breaks connectivity with
11g + CTS protected connections. Add a comment to the code why we don't use 10us
for CCK SIFS value.

The OFDM SIFS value is set to 16us (as defined in 802.11 and also used by the
Ralink drivers).

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Acked-by: Ivo van Doorn <IvDoorn@gmail.com>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>